Pretty sure online in-game chat (and definitely not any specific online service) isn't to blame for racist, sexist, homophobic, whatever rants. Anything online basically guarantees anonymity and the ability to say just about anything without fear of reprisal, and everyone knows this. And there's just something about FPS games that get players riled up the worst when they're getting schooled, so naturally the response is to just yell out the first thing that comes to mind without any sort of imagination or originality whatsoever.
But you know what they say about the biggest homophobes.
The funniest reply I've heard on XBL to someone screaming "FAG!"
"That's right, I am a fag, and I'm about to f*ck you up the @ss!"